I've had a pc that wouldn't post until I pulled the hdd. Then it would post, boot from a cd, and ultimately from a new hdd.
Probably the PSU then.
Get your Multimeter out and measure the outputs of the 12v and 5v rails on the PSU motherboard connector and see what it is giving out.
...with a paperclip shorting pins 15 & 16 (green and black) natch
Will check at the weekend.
I got drunk and forgot all about it?